There exists a variety of methods of welding or cutting materials, especially metals, where the process for practical reasons is desired to be carried out in air, yet the hot (particularly the molten) metal is desired to be protected from the air, and/or from contamination (e.g. dust). Such systems commonly use arc or other plasma heating. The requirement for protecting hot metal (or electrodes) from the air while operating in air is accomplished by the use of an approximately cylindrically shaped nozzle or cup that fits over the heat source (e.g. contact tubes or electrodes) such that an inert gas can be passed through the nozzle and over the electrodes and the hot metal to protect them from the air. Such cups or nozzles are mainly or exclusively metal or ceramic; the latter being extensively used for TIG (tungsten inert gas) welding since the amount of heating and especially metal spattering onto the nozzle is limited.
Ceramics have been tried for MIG welding in an attempt to reduce spatter buildup on the nozzle (a more severe problem with MIG welding than TIG welding) and hence thereby minimize diversion or blockage of gas flow. Ceramic nozzles have also been used because they are electrical insulators and are lighter weight than corresponding metal nozzles. Nevertheless, ceramic nozzles have been generally unsuccessful for MIG welding because of greater spatter problems or mechanical failure (due to thermal stresses or mechanical impact) or both. Further, some monolithic ceramics, e.g. silicon nitride (Si.sub.3 N.sub.4), that have shown a degree of technical success, have proved far too expensive (e.g. by 5-10 fold).
This invention addresses both the performance and cost issues by combining a set of technologies to give a family of nozzles to meet a range of cost and performance criteria.
The patent literature is replete with attempts (largely unsuccessful) to solve MIG welding cup problems of slagging (slag or spatter buildup), fracture, and erosion. The following U.S. patents may be mentioned.
U.S. Pat. No. 2,827,550 discloses a cup (uncoated) made of a thermoset resin reinforced with paper, glass fiber, or asbestos.
In U.S. Pat. No. 3,237,648 the contact tube (composition unstated) is coated with silicon nitride. The cup (composition unstated) is not coated.
U.S. Pat. No. 3,430,837 discloses a ceramic cup and center tube coated with Teflon or pyrolytic graphite.
U.S. Pat. No. 3,536,888 discloses a cup formed of porcelain, alumina, beryllia, zirconium silicate, zirconia, magnesium aluminum silicate, cordierite, mullite, ceramic graphite, or boron nitride. The cup is coated with a silicate or silicone.
In U.S. Pat. No. 4,575,612 the cup comprises a refractory steel sheath coated with chromium oxide, alumina, or "ceramic". The contact tube may comprise braided glass fiber coated with silicone.
U.S. Pat. No. 4,672,163 discloses an uncoated cup formed of silicon nitride or sialon.